The present invention relates to an optical lithographic technique of forming a resist pattern in semiconductor device fabrication and more particularly, a pattern size evaluation apparatus for evaluating a resist pattern during or after developing.
In order to form an integrated circuit pattern on a wafer, a desired pattern is projected on resist on exposure after resist is applied on the wafer, the resist is developed to a resist pattern and the resist pattern is used as a mask to perform processing such as etching or the like. A resist pattern as a mask is required a high size accuracy and a size of the resist pattern is changed in dependence on elapsed time in developing to an great extent. Therefore, control of a developing time is very important.
Conventionally, size control in developing has been performed with a constant developing time. That is, at first, a preceding wafer is exposed to light and developed in order to determine an optimum developing time. Then, all the other wafers are developed with the optimum developing time as a constant time.
However, in actual cases, a history of each wafer in process, such as a post exposure delay and the like, is different from another. Therefore, if wafers of the same lot are subjected to developing in the same time, a dispersion in size between the wafers arises, so that developing cannot be terminated with a desired size for every wafer.
Though such control for developing time has so far been adopted, there has arisen no severe problem since an error in size is within an allowable range.
However, nowadays, miniaturization in the process is progressed and more accurate size control has been required, which makes size control in developing harder, if with the conventional means, for controlling a developing time.
On the other hand, evaluation of a resist pattern after developing has conventionally performed by, for example, SEM (Scanning Electron Microscope). While SEM enables pattern observation under high magnification, it is complex in apparatus structure and expensive. Moreover, since it takes a very long time for inspection, it is harder to be efficient in evaluation.
In the cases where a device pattern is optically inspected by use of diffracted light and the like during developing or after developing, information from an inspected pattern sometimes includes information from another pattern in monitoring, which has been a factor to make accuracy in monitoring deteriorated.
The present invention has been made in consideration of the above mentioned circumstances and it is an object of the present invention to provide a pattern size evaluation apparatus, which makes it possible to perform size control of a resist pattern on a different wafer with high accuracy.
It is another object to provide an pattern size evaluation apparatus which makes it possible to inspect a resist pattern in a short time with high accuracy without reception of an influence of a pattern other than a pattern to be monitored.
It is a further object to provide an pattern size evaluation apparatus which makes it possible to evaluate a resist pattern during developing in a short time with high accuracy and to predict an end point of developing with high accuracy.
In order to solve the above problems, a pattern size evaluation apparatus comprising: an illumination optical system for projecting parallel light rays of a predetermined wavelength on a monitoring area formed at a position different from a device pattern on an object; a light intensity detection optical system for detecting diffracted light from the monitoring area; and a device pattern size evaluation section for evaluating a size of the device pattern based on an intensity of diffracted light from the monitoring area.
The apparatus can be applied for evaluation of a device pattern size after developing is terminated or applied for detection of an end point of developing of the device pattern. Where the apparatus is applied to detect an end point of developing of a device pattern, the apparatus comprises a section for judging the end point of developing based on a change in intensity of diffracted light from the monitoring area.
As the monitoring area, in the first place, a monitoring area, which comprises a monitoring pattern, is preferred. The monitoring pattern comprises an element pattern different from a device pattern so that diffracted light from the monitoring pattern is detected by being separated from diffracted light of the device pattern. The monitoring pattern is preferably a pattern, through which resist is exposed, and which comprises elements thereof are in a proximity relation periodically disposed so that a distance between adjacent pairs of the elements is equal to or less than the limit of resolution.
In this case, it is only required that pattern size evaluation section performs pattern evaluation based on a relation obtained in advance between the size of a device pattern and the intensity of diffracted light from the monitoring pattern.
In the second place, the monitoring area may be formed by exposing such that the monitoring area will be etched at a uniform developing speed across the whole area and decrease in fi thickness gradually.
In this case, the monitoring area is formed by being exposed in the same condition as that of the device pattern. That is, the mask pattern for forming the monitoring area is formed on the same plane as a mask patter for device pattern is formed. The mask pattern for forming the monitoring area may be a pattern with a pitch thereof which reaches the object only with diffracted light of order 0 in the exposure condition. Then a ratio in area between transmission area and block area in the mask pattern for forming the monitoring area is designed so that decrease in film thickness arises gradually in a uniform manner in company with progress of developing.
In this case, as illumination light projected on the monitoring area from illumination system, it is preferred that the light have a wavelength with which a change in intensity of diffracted light has maximums and minimums in the course of decrease in film thickness of the monitoring area. And the section for predicting an end point of the developing predicts the end point of developing based on detected maximums and minimums of diffracted light intensity, or detected maximum and minimum of a value of the derivative of a change in diffracted light intensity with respect to time.
Further, as a monitoring area, in the third place, the area may be a monitoring region in which exposure is performed so that a film thickness is to be a predetermined value with a timing determined based on an end point of developing for a device pattern. In this case, it is only required that means for predicting an end point of developing is to determined the end point of developing by detecting the timing when the film thickness becomes the predetermined value.
According to the present invention, a size inspection of device pattern is performed with use of a monitoring area comprising an element which is discernible from a device pattern and an already processed pattern.
In the case where a monitoring pattern is formed in a monitoring area by exposure, for example, a circle or a polygon is used as an element pattern of an exposure mask for forming the monitoring pattern, and a distance between adjacent element patterns is designed so as to be equal to or less than the limit of resolution in exposure to light. With a monitoring pattern formed in such a manner, a change thereof in intensity of diffracted light is larger than that from a device pattern and thereby accuracy in inspection can be improved.
If a pitch, a monitoring pattern of a direction of repetition thereof or the like being different, is used for evaluation of a pattern, a inspection with high accuracy, which is not affected by another pattern, can be performed. Moreover, since a pattern evaluation can be performed only by monitoring an intensity of diffracted light, an inspection can be conducted in a short time, which is different from SEM and the like.
In the case where as a monitoring area, a monitoring area is used which has no pattern as mentioned above, which is exposed so that a almost uniform developing speed is achieved across the whole surface, and with which an almost uniform decrease in film thickness is achieved as developing progresses, maximums and minimums of an intensity of diffracted light from the monitoring area can be observed. An end point of developing can be predicted based on the maximums and minimums.
According to such a prediction method, even when an offset in terms of a sign to be attached to an absolute value of an intensity signal of diffracted light arises, a correct end point of developing can be obtained by correcting the offset, since it is so arranged that an end point in developing is predicted based on maximums or minimums.
Additional objects and advantages of the invention will be set forth in the description which follows, and in part will be obvious from the description, or may be learned by practice of the invention. The objects and advantages of the invention may be realized and obtained by means of the instrumentalities and combinations particularly pointed out in the appended claims.